This disclosure relates generally to spectrally dispersive optical systems. More particularly, this disclosure relates to spectrometers utilizing two-material prism elements.
Numerous optical systems utilize the dispersion of light into constituent wavelength bands. Such dispersion is used for many purposes, including but not limited to spectrometry or other spectral analysis. The dispersion of light is conventionally achieved through a dispersive element, such as a diffraction grating or a prism.
Diffraction grating dispersive elements, which include single blaze-angle or multi-faceted (i.e. dual-blaze angle) gratings, utilize the periodic nature of the constituent material to separate out different wavelength bands of light as it intersects the blazing of the grating. Prisms, on the other hand, utilize the transition between materials, such as between air and the solid of the prism, to disperse the incident light. Prisms generally have the advantage of yielding a higher total throughput via their dispersion than a grating achieves through diffraction, particularly when operating over a spectral range that includes more than one octave (here an octave refers to a doubling in frequency). However, prisms may be more prone to certain distortions, as discussed below.
A prism utilizing only a single material yields a dispersion curve proportional to the change in the material's index of refraction versus wavelength. Typically this change in dispersion with wavelength is very large and acts as an unacceptable distortion. To counteract the unwanted distortion of some prisms, multiple elements of different materials (conventionally different types of glass) and/or different shapes may be joined and utilized to control the dispersion of different wavelength bands to reach a desired optical effect. The concept of utilizing multiple materials in prisms utilizes a high dispersion material (i.e. a “flint” glass) with a low dispersion material (i.e. a “crown” glass), to form an achromatic pair that allows control of the dispersion of different wavelengths of light, which may be optimized for particular optical systems. For example, two-material prisms may be configured to correct for undesired dispersion effects in the optical system. With prisms, one common undesired dispersion effect that may arise is a large variation from linearity of the spectral dispersion, which may result from the differing refractive indexes associated with different wavelengths of light in the optical elements.
In various situations, desired optical system specifications may prefer selection of systems that meet a particular spectral sampling interval (the spacing between sample points in a measured spectra). To achieve the desired sampling interval, the dispersive element may be configured to limit variance in detector bandwidth over a wide band of wavelengths. Such limited variance may be characterized by the system having a relatively linear or “flat” dispersion. To achieve relative flatness of dispersion in prism-based dispersive elements, designers may select from a large number of flint and crown materials, to construct a two-material achromatic prism. As one example of conventional material selection, some optical systems may utilize a two-glass prism having a fused quartz crown (an amorphous/glassy form of quartz, such as that distributed under the trademark INFRASIL®) and an SF1 flint (where SF1 is the SCHOTT glass designation for a common flint glass). Thus far, material selection for conventional two-material achromatic prisms for spectrometers have involved combining glassy materials based on empirical observations of their properties, and refining material selections based on the effectiveness of the combination.
What is needed is, among other things, improvements over known dispersive elements for spectroscopic purposes, wherein their constituent materials provide reduced variance of dispersion over a wide wavelength band.